The present invention relates to a control system for an electromagnetic clutch for an automatic transmission of a motor vehicle.
An automobile provided with a continuously variable belt-drive transmission with an electromagnetic clutch is disclosed in EP-A 151038. The electromagnetic clutch of the transmission is controlled by a control system to provide various operational modes for clutch torque (clutch current). One of the modes is selected in accordance with a position of a selector lever and driving conditions of the vehicle to control the electromagnetic clutch.
At a start of the vehicle, the clutch torque is raised in proportion to the engine speed or the opening degree of a throttle valve of the engine. When the vehicle speed reaches a predetermined speed, the clutch is entirely engaged by a lock-up current. When the vehicle speed decreases below a predetermined speed, the clutch is disengaged to prevent stalling of the engine. Generally, the vehicle speed is detected by counting pulses proportional to the speed.
On the other hand, when the vehicle speed is suddenly braked by quickly and greatly depressing a brake pedal, the deceleration of the vehicle becomes very large. When the vehicle is decelerated, the pulse separation of the pulses becomes long. As a result, the detection of the predetermined speed for disengaging the clutch is delayed.
Thus, the disengagement of the clutch is delayed, causing a decrease of engine speed or stalling of the engine because of overload on the engine. Particularly, on a snowy road, wheels are liable to be rapidly locked by braking, delaying the disengagement of the clutch.
In order to solve the problem, it may be preferable to set the predetermined vehicle speed for disengaging the clutch at a high speed. However, in such a system, since the clutch is disengaged at the high vehicle speed, the engine braking effect reduces.